Related
In C++, if I want to have a member of a class that has no default constructor, I need to explicitly initialize it like this:
class a
{
a(int)
{
}
};
class b
{
a x;
b() : x(3)
{
}
};
However, sometimes the initialization for, say, x, could not be so simple. It could either be a very very complicated formula or a whole, algorithmic function with fors and ifs that needs to be computed. I usually solve it like this:
class b
{
a * x;
b()
{
// Do some very complicated computations
this->x = new a(result_of_complicated_computations);
}
};
Which, however, forces me to delete the pointer at the moment of destruction, it slightly less performing and makes my code somehow untidy and ugly. I am really surprised that there is no way to compute parameters for constructors of members within the constructor of the class, without having to use this kind of tricks.
Are there more elegant solutions to this?
Use a helper function:
class b
{
a x;
b() : x(complicated_computation()) { }
private:
static int complicated_computation() {
return result_of_complicated_computations;
}
};
And the default-constructor isn't deleted, but undeclared.
Do the computations in a function:
static a complicated_computations();
b() : x(complicated_computations()) {}
I don't get it. If you have:
class A
{
private:
//a lot of members
public:
A(/* complex object used for construction */)
: //initialize members
{ }
}
You can simply do:
class B
{
private:
A a;
public:
B()
: a(call_to_function_that_generates_this_complex_data())
{ }
};
You can even use your member functions during construction. But note, that:
ยง 12.6.2.13:
Member functions (including virtual member functions,
10.3) can be called for an object under construction.(...) However, if these operations are performed in a ctor-initializer (or in a function
called directly or indirectly from a ctor-initializer) before all the
mem-initializers for base classes have completed, the result of the
operation is undefined.
I have the following C++-class:
// Header-File
class A
{
public:
A();
private:
B m_B;
C m_C;
};
// cpp-File
A::A()
: m_B(1)
{
m_B.doSomething();
m_B.doMore();
m_C = C(m_B.getSomeValue());
}
I now would like to avoid the class A to call any constructor of C m_C. Because on the last line in A::A(), I'm anyways going to initialize m_C myself because I need to prepare m_B first. I could provide an empty default constructor for class B. But that's not the idea.
I have already tried to add m_C(NULL) to the init-list of A::A(). Sometimes it worked, sometimes it said there was no constructor taking NULL as an argument.
So how can I have m_C left uninitialized? I know that with pointers, the m_C(NULL)-way works. And I don't want to allocate it dynamically using new.
Any idea is appreciated.
How about using technique described in this QA?
Prevent calls to default constructor for an array inside class
std::aligned_storage<sizeof(T[n]), alignof(T)>::type
Or, you also can consider using of union. AFAIK, unions will be initialized only with first named member's constructor.
For example,
union
{
uint8_t _nothing = 0;
C c;
};
According to the standard mentioned in the QA, c will be zero-initialized, and its constructor will not be called.
You can't.
All member variables are full constructed when the construcotr code block is entered. This means there constructors must be called.
But you can work around this restriction.
// Header-File
class A
{
struct Initer
{
Initer(B& b)
: m_b(b)
{
m_b.doSomething();
m_b.doMore();
}
operator int() // assuming getSomeValue() returns int.
{
return m_b.getSomeValue();
}
B& m_b;
};
public:
A();
private: // order important.
B m_B;
C m_C;
};
// cpp-File
A::A()
: m_B(1)
, m_C(Initer(m_B))
{
}
I don't see a good way to achieve what you want. This must be a workaround:
// Header-File
class A
{
public:
A();
private:
B m_B;
C m_C;
static int prepareC(B& b);
};
// cpp-File
A::A()
: m_B(1)
, m_C(prepareC(m_B))
{
}
int A::prepareC(B& b)
{
b.doSomething();
b.doMore();
return b.getSomeValue();
}
Please ensure that m_B.doSomething(), m_B.doMore() and m_B.getSomeValue() don't touch m_C (directly or indirectly).
As #Tobias correctly mentions, this solution depends on the order of initialization. You need to ensure that the definitions of m_B and m_C are in this order.
Updated the code according to #Loki's idea.
What you ask is forbidden - and correctly so. This ensures that every member is correctly initialized. Do not try to work around it - try to structure your classes that they work with it.
Idea:
C has a constructor that does nothing
C has an initialization method that makes the class usable
C tracks whether it has been initialized correctly or not and returns appropriate errors if used without initialization.
The pointer sounds like the only clean solution to me. The only other solution I see is to have a default constructor for C that does nothing and have an initialising method in C you call yourself later.
m_C.Initialise( m_B.getSomeValue() );
Easiest is storing pointers to a B and a C. These can be initialized to 0, omitting any construction. Be careful not to dereference a null pointer and delete it in the destructor of A (or use std::unique_ptr/boost::scoped_ptr).
But why not initialize m_B first (through a proper constructor call, not in A::A(), and then use that initialized B instance to initialize m_C? It will call for a small rewrite, but I bet it'll be worth the code cleanup.
If you don't want to allocate it dynamically using new for code clutter/exception safety reasons, you can use a std::unique_ptr or std::auto_ptr to solve this problem.
A solution that avoids new is to edit C to have a two-step initialization process. The constructor would then construct a "zombie" object, and you'd have to call an Initialize method on that m_C instance to finish your initialization. This is similar to the existing cases you found where you could pass NULL to the constructor, and later go back to initialize the object.
Edit:
I thought of this earlier (even though it looks much like other people's solutions). But I had to get some confirmation that this wouldn't break before I added this solution - C++ can be quite tricky, and I don't use it very often :)
This is cleaner than my other suggestions, and doesn't require you to mess with any implementation but that of A.
Simply use a static method as the middle-man on your initialization:
class A
{
public:
A();
private:
static int InitFromB(B& b)
{
b.doSomething();
b.doMore();
return b.getSomeValue();
}
// m_B must be initialized before m_C
B m_B;
C m_C;
};
A::A()
: m_B(1)
, m_C(InitFromB(m_B))
{
}
Note that this means you can't allow m_B to depend on the instance of A or C at all, whereas the solutions at the top of this answer might allow you to pass A or m_C into m_B's methods.
Just use comma expressions:
A::A()
: m_B(1)
, m_c(m_B.doSomething(), m_B.doMore(), m_B.getSomeValue())
{
}
Obviously, as others have explained, m_B better be declared before m_C else m_B.doSomething() invokes undefined behavior.
Here we have the building blocks:
#include <iostream>
class C
{
public:
C(int i){std::cout << "C::C(" << i << ")" << std::endl;}
};
class B
{
public:
B(int i){std::cout << "B::B(" << i << ")" << std::endl;}
void doSomething(){std::cout << "B::doSomething()" << std::endl;}
void doMore(){std::cout << "B::doMore()" << std::endl;}
int getSomeValue(){return 42;}
};
If you want to make a new kind of construction for B consider making a derived class:
class B1 : public B
{
public:
B1() : B(1)
{
doSomething();
doMore();
}
};
Now use the class B1 that is derived from B:
class A
{
private:
B1 _b;
C _c;
public:
A() : _c(_b.getSomeValue()){std::cout << "A::A()" << std::endl;}
};
And then:
int main()
{
A a;
}
Output:
B::B(1)
B::doSomething()
B::doMore()
C::C(42)
A::A()
class A;
class B {
public:
B(A& a) : a(a) {}
private:
A& a;
};
/* Method 1 */
/* warning C4355: 'this' : used in base member initializer list */
/*
class A {
public:
A() : b(*this) {}
private:
B b;
};
*/
/* Method 2 */
/* But I need to manually perform memory dellocation. */
class A {
public:
A() { b = new B(*this); }
~A() { delete b; }
private:
B* b;
};
int main() {
}
Currently, when I try to initialize the reference in B, I am using Method 1. However, Method 1 will flag me warning which is understandable.
Hence, I have to fall back using Method 2, by using dynamic memory allocation.
Is there any better way I can use, without the need of manual memory allocation/ dellocation (OK. I know smart pointer)?
I prefer Method 1, just that I am not comfortable with the warning.
Note this is a warning (so it is dangerous not illegal).
What the compiler is worried about is that you are passing a pointer for an object that has not been fully initialized. Thus if the pointer is used in the B class constructor you are in undefined behavior.
So if you do use this the only thing you can do is assign the pointer to a member variable (reference or pointer). But note be careful of the assignment to a variable as you may invoke an implicit cast (I am not sure if that is actually a problem but the RTTI is not available until the object is fully formed).
What are you trying to achieve by storing the reference?
Doing this is valid.
However, you must ensure (I mean by yourself, there's no way the compiler can do this) that the this is not used to call virtual functions until the object is fully constructed.
Depending on what you're doing, a method might be to factor out the parts of A that B needs, than have A inherit from the part.
struct bar_base; // interface foo wants
struct foo
{
foo(bar_base& pX) :
mX(pX)
{}
bar_base& mX;
};
struct bar_base
{
/* whatever else */
protected:
bar_base& get_base(void)
{
// getting `this` went here; safe because bar_base is initialized
return *this;
}
};
struct bar : bar_base
{
bar(void) :
// bar_base is already initialized, so:
mX(get_base())
{}
foo mX;
};
Obviously, this depends on what you're doing. This makes sure you never get undefined behavior.
But really, it's just warning. If you promise to never use this in B's constructor, you're fine, and can silence the warning this way:
struct bar;
struct foo
{
foo(bar& pX) :
mX(pX)
{}
bar& mX;
};
struct bar
{
bar(void) :
mX(self())
{}
foo mX;
private:
bar& self(void)
{
// fools the warning
return *this;
}
};
Make sure you know what you're doing, though. (Perhaps it could be re-designed?)
Well, one obvious way to avoid the warning is to make B store a pointer-to-A, then you don't have to initialise it in B's constructor/A's initialiser list, and can wait until the body of A's constructor is executing....
Take this warning seriously. Your this object is not yet fully constructed and passing around is not safe (if you ever accidentally call a function on this you invoke UB). Also, there are other techniques for memory management. Try looking up the STL design of allocators.
You could also use RAII/smart pointers to achieve the same effect.
Or, are you trying to write a garbage collector/memory profiler of sorts?
class A{
A(int a = 5){
DoSomething();
A();
}
A(){...}
}
Can the first constructor call the second one?
Not before C++11.
Extract the common functionality into a separate function instead. I usually name this function construct().
The "so-called" second call would compile, but has a different meaning in C++: it would construct a new object, a temporary, which will then be instantly deleted at the end of the statement. So, no.
A destructor, however, can be called without a problem.
Not before C++0x, no.
BUT, just out of academic interest I've come up with a really horrible way* to do it using a placement operator "new" (someone care to point out how portable this is?)
#include <new>
#include <iostream>
class A
{
public:
A(int i, int j)
: i_(i), j_(j) { }
A(int i)
{ new (this) A(i, 13); }
int i_,j_;
};
int
main() {
A a1(10,11), a2(10);
std::cout
<< a1.i_ << ", "
<< a1.j_ << std::endl
<< a2.i_ << ", "
<< a2.j_ << std::endl;
return 0;
}
*Hell no, I don't write this in the production code.
The answer is in fact "yes", but as others have suggested, it doesn't do what you want. You can of course use the constructor of a base class, either implicitly or explicitly:
struct B {
B() {}
B( int x ) {}
};
struct A : public B {
A() {} // calls B() implicitly
A( int a, int b ) : B( b ) {} // calls B(int) explicitly
};
Not directly. There are a few ways to work around this.
From the initializer list of your class' constructor, you can call a constructor on any base class, and on all member variables.
So you can usually refactor your class and split it into several smaller ones to solve the problem. The commonly executed code can be placed in a member object or perhaps a base class. Then each of the main class' constructors just have to decide which construcotr to use to initialize that member.
class B {
B() { }
B(int b) { DoSomething(); }
}
class A{
A(int a = 5) : b(a) { } // call B's constructor which does something
A() : b() {} // call B's constructor which does nothing
B b;
};
This is an old question; however,
class A{
A(int a = 5){
DoSomething();
A();
}
A(){...}
}
could be
class A{
A(int a = 5){
*this = A();
DoSomething();
}
A(){...}
}
As pointed out by Pavel Radzivilovsky in his answer, since C++ 11, it is possible. It is the same syntax as for explicitely calling the parent's class constructor from a child class. This is useful when a class needs to have multiple constructors (say, a default constructor and a constructor with attribute initialization) but some operations have to be done in all cases. This allows to avoid code repetitions.
Here is an example:
class A
{
public:
A()
{
foo();
}
A(Attribute attribute) : A()
{
this->attribute = attribute;
}
//------ some other code --------
private:
Attribute attribute;
void foo()
{...}
//------ some other code -------
};
In this simple example, I assume that the function foo() needs to be called in all cases for the object to be correctly initialized. With this syntax, if the second constructor (with attribute initialization) is called, it will first perform the operations in the default constructor before executing the instructions in the attribute-initialization constructor.
It can also be done the other way around: the default constructor can call another constructor with default parameters.
Before C++ 11, it was necessary to duplicate the common instructions of all constructors or define methods that do the actual object initialization.
As a C# developer I'm used to running through constructors:
class Test {
public Test() {
DoSomething();
}
public Test(int count) : this() {
DoSomethingWithCount(count);
}
public Test(int count, string name) : this(count) {
DoSomethingWithName(name);
}
}
Is there a way to do this in C++?
I tried calling the Class name and using the 'this' keyword, but both fail.
C++11: Yes!
C++11 and onwards has this same feature (called delegating constructors).
The syntax is slightly different from C#:
class Foo {
public:
Foo(char x, int y) {}
Foo(int y) : Foo('a', y) {}
};
C++03: No
Unfortunately, there's no way to do this in C++03, but there are two ways of simulating this:
You can combine two (or more) constructors via default parameters:
class Foo {
public:
Foo(char x, int y=0); // combines two constructors (char) and (char, int)
// ...
};
Use an init method to share common code:
class Foo {
public:
Foo(char x);
Foo(char x, int y);
// ...
private:
void init(char x, int y);
};
Foo::Foo(char x)
{
init(x, int(x) + 7);
// ...
}
Foo::Foo(char x, int y)
{
init(x, y);
// ...
}
void Foo::init(char x, int y)
{
// ...
}
See the C++FAQ entry for reference.
Yes and No, depending on which version of C++.
In C++03, you can't call one constructor from another (called a delegating constructor).
This changed in C++11 (aka C++0x), which added support for the following syntax:
(example taken from Wikipedia)
class SomeType
{
int number;
public:
SomeType(int newNumber) : number(newNumber) {}
SomeType() : SomeType(42) {}
};
I believe you can call a constructor from a constructor. It will compile and run. I recently saw someone do this and it ran on both Windows and Linux.
It just doesn't do what you want. The inner constructor will construct a temporary local object which gets deleted once the outer constructor returns. They would have to be different constructors as well or you would create a recursive call.
Ref: https://isocpp.org/wiki/faq/ctors#init-methods
C++11: Yes!
C++11 and onwards has this same feature (called delegating constructors).
The syntax is slightly different from C#:
class Foo {
public:
Foo(char x, int y) {}
Foo(int y) : Foo('a', y) {}
};
C++03: No
It is worth pointing out that you can call the constructor of a parent class in your constructor e.g.:
class A { /* ... */ };
class B : public A
{
B() : A()
{
// ...
}
};
But, no, you can't call another constructor of the same class upto C++03.
In C++11, a constructor can call another constructor overload:
class Foo {
int d;
public:
Foo (int i) : d(i) {}
Foo () : Foo(42) {} //New to C++11
};
Additionally, members can be initialized like this as well.
class Foo {
int d = 5;
public:
Foo (int i) : d(i) {}
};
This should eliminate the need to create the initialization helper method. And it is still recommended not calling any virtual functions in the constructors or destructors to avoid using any members that might not be initialized.
If you want to be evil, you can use the in-place "new" operator:
class Foo() {
Foo() { /* default constructor deliciousness */ }
Foo(Bar myParam) {
new (this) Foo();
/* bar your param all night long */
}
};
Seems to work for me.
edit
As #ElvedinHamzagic points out, if Foo contained an object which allocated memory, that object might not be freed. This complicates things further.
A more general example:
class Foo() {
private:
std::vector<int> Stuff;
public:
Foo()
: Stuff(42)
{
/* default constructor deliciousness */
}
Foo(Bar myParam)
{
this->~Foo();
new (this) Foo();
/* bar your param all night long */
}
};
Looks a bit less elegant, for sure. #JohnIdol's solution is much better.
Simply put, you cannot before C++11.
C++11 introduces delegating constructors:
Delegating constructor
If the name of the class itself appears as class-or-identifier in the
member initializer list, then the list must consist of that one member
initializer only; such constructor is known as the delegating
constructor, and the constructor selected by the only member of the
initializer list is the target constructor
In this case, the target constructor is selected by overload
resolution and executed first, then the control returns to the
delegating constructor and its body is executed.
Delegating constructors cannot be recursive.
class Foo {
public:
Foo(char x, int y) {}
Foo(int y) : Foo('a', y) {} // Foo(int) delegates to Foo(char,int)
};
Note that a delegating constructor is an all-or-nothing proposal; if a constructor delegates to another constructor, the calling constructor isn't allowed to have any other members in its initialization list. This makes sense if you think about initializing const/reference members once, and only once.
No, in C++ you cannot call a constructor from a constructor. What you can do, as warren pointed out, is:
Overload the constructor, using different signatures
Use default values on arguments, to make a "simpler" version available
Note that in the first case, you cannot reduce code duplication by calling one constructor from another. You can of course have a separate, private/protected, method that does all the initialization, and let the constructor mainly deal with argument handling.
Another option that has not been shown yet is to split your class into two, wrapping a lightweight interface class around your original class in order to achieve the effect you are looking for:
class Test_Base {
public Test_Base() {
DoSomething();
}
};
class Test : public Test_Base {
public Test() : Test_Base() {
}
public Test(int count) : Test_Base() {
DoSomethingWithCount(count);
}
};
This could get messy if you have many constructors that must call their "next level up" counterpart, but for a handful of constructors, it should be workable.
In Visual C++ you can also use this notation inside constructor: this->Classname::Classname(parameters of another constructor). See an example below:
class Vertex
{
private:
int x, y;
public:
Vertex(int xCoo, int yCoo): x(xCoo), y(yCoo) {}
Vertex()
{
this->Vertex::Vertex(-1, -1);
}
};
I don't know whether it works somewhere else, I only tested it in Visual C++ 2003 and 2008. You may also call several constructors this way, I suppose, just like in Java and C#.
P.S.: Frankly, I was surprised that this was not mentioned earlier.
This approach may work for some kinds of classes (when the assignment operator behaves 'well'):
Foo::Foo()
{
// do what every Foo is needing
...
}
Foo::Foo(char x)
{
*this = Foo();
// do the special things for a Foo with char
...
}
I would propose the use of a private friend method which implements the application logic of the constructor and is the called by the various constructors. Here is an example:
Assume we have a class called StreamArrayReader with some private fields:
private:
istream * in;
// More private fields
And we want to define the two constructors:
public:
StreamArrayReader(istream * in_stream);
StreamArrayReader(char * filepath);
// More constructors...
Where the second one simply makes use of the first one (and of course we don't want to duplicate the implementation of the former). Ideally, one would like to do something like:
StreamArrayReader::StreamArrayReader(istream * in_stream){
// Implementation
}
StreamArrayReader::StreamArrayReader(char * filepath) {
ifstream instream;
instream.open(filepath);
StreamArrayReader(&instream);
instream.close();
}
However, this is not allowed in C++. For that reason, we may define a private friend method as follows which implements what the first constructor is supposed to do:
private:
friend void init_stream_array_reader(StreamArrayReader *o, istream * is);
Now this method (because it's a friend) has access to the private fields of o. Then, the first constructor becomes:
StreamArrayReader::StreamArrayReader(istream * is) {
init_stream_array_reader(this, is);
}
Note that this does not create multiple copies for the newly created copies. The second one becomes:
StreamArrayReader::StreamArrayReader(char * filepath) {
ifstream instream;
instream.open(filepath);
init_stream_array_reader(this, &instream);
instream.close();
}
That is, instead of having one constructor calling another, both call a private friend!
If I understand your question correctly, you're asking if you can call multiple constructors in C++?
If that's what you're looking for, then no - that is not possible.
You certainly can have multiple constructors, each with unique argument signatures, and then call the one you want when you instantiate a new object.
You can even have one constructor with defaulted arguments on the end.
But you may not have multiple constructors, and then call each of them separately.
When calling a constructor it actually allocates memory, either from the stack or from the heap. So calling a constructor in another constructor creates a local copy. So we are modifying another object, not the one we are focusing on.
Would be more easy to test, than decide :)
Try this:
#include <iostream>
class A {
public:
A( int a) : m_a(a) {
std::cout << "A::Ctor" << std::endl;
}
~A() {
std::cout << "A::dtor" << std::endl;
}
public:
int m_a;
};
class B : public A {
public:
B( int a, int b) : m_b(b), A(a) {}
public:
int m_b;
};
int main() {
B b(9, 6);
std::cout << "Test constructor delegation a = " << b.m_a << "; b = " << b.m_b << std::endl;
return 0;
}
and compile it with 98 std:
g++ main.cpp -std=c++98 -o test_1
you will see:
A::Ctor
Test constructor delegation a = 9; b = 6
A::dtor
so :)